1. Field of the Invention
The present invention relates to band pass filters, and more particularly, the present invention relates to band pass filters included in mobile communication devices such as cellular phones, and various electronic apparatuses.
2. Description of the Related Art
In general, this kind of band pass filter has a plurality of LC resonators, which perform a mutual inductive coupling, which is the so-called xe2x80x9cMxe2x80x9d coupling, and a capacitive coupling between the LC resonators. FIG. 8 shows an electric equivalent circuit diagram of a two-stage band pass filter having two LC resonators. In this band pass filter, an LC resonator 1 is connected to an input terminal IN via a coupling capacitor C5, and an LC resonator 2 is connected to an output terminal OUT via a coupling capacitor C6. The LC resonator 1 is defined by a parallel circuit including an inductor L1 and a capacitor C1. The LC resonator 2 is defined by a parallel circuit including an inductor L2 and a capacitor C2. The LC resonator 1 and the resonator 2 achieve a mutual inductive coupling between the LC resonators 1, 2.
FIG. 9 shows the detailed structure of a conventional band pass filter 80 having the above equivalent circuit, and FIG. 10 shows an example of the appearance of the conventional band pass filter 80. As shown in FIG. 9, the band pass filter 80 includes a ceramic sheet 63 having an input lead pattern 73 and an output lead pattern 74 disposed on a surface thereof, a ceramic sheet 64 having inductor patterns 69 and 70 disposed on a surface thereof, a ceramic sheet 65 having capacitor patterns 71 and 72 disposed on a surface thereof, a ceramic sheet 61 having a shield electrode 75 disposed on a surface thereof, a ceramic sheet 67 having a shield electrode 76 disposed on a surface thereof.
The inductor L1 is defined by the inductor pattern 69, and the inductor L2 is defined by the inductor pattern 70. The capacitor C1 is defined by the capacitor pattern 71 and an open end 69b of the inductor pattern 69, which opposes the capacitor pattern 71. The capacitor C2 is defined by the capacitor pattern 72 and an open end 70b of the inductor pattern 70, which opposes the capacitor pattern 72. The coupling capacitor C5 is defined by the inductor pattern 69 and the input lead pattern 73. The coupling capacitor C6 is defined by the inductor pattern 70 and the output lead pattern 74.
Lead portions 69a and 70a of the inductor patterns 69 and 70 respectively provided on the surface of the ceramic sheet 64 are exposed at the front and back surfaces of the ceramic sheet 64. In addition, the capacitor patterns 71 and 72 respectively provided on the surface of the ceramic sheet 65 are exposed at the back and front surfaces of the ceramic sheet 65. That is, the inductor pattern 69 of the LC resonator 1 and the inductor pattern 70 of the LC resonator 2 are arranged opposite to each other, and the capacitor pattern 71 of the LC resonator 1 and the capacitor pattern 72 of the LC resonator 2 are arranged opposite to each other. As a result, the LC resonator 1 and the LC resonator 2 perform an interdigital coupling between the LC resonators 1, 2.
As shown in FIG. 10, an input terminal IN, an output terminal OUT, and ground terminals G1 and G2 are provided on a laminated body 78 defined by laminating the ceramic sheets 61 to 68. The input terminal IN is connected to the input lead pattern 73, and the output terminal OUT is connected to the output lead pattern 74. The ground terminal G1 is connected to the lead portion 69a of the inductor pattern 69, the lead portion 72a of the capacitor pattern 72, ends 75a of the shield electrode 75, and ends 76a of the shield electrode 76. The ground terminal G2 is connected to the lead portion 70a of the inductor pattern 70, the lead portion 71a of the capacitor pattern 71, the other ends 75b of the shield electrode 75, and the other ends 76b of the shield electrode 76.
In the conventional band pass filter 80, the inductor patterns 69 and 70 are located on the same ceramic sheet 64, and the capacitor patterns 71 and 72 are located on the same ceramic sheet 65. The inductor patterns 69 and 70 extend to the mutually opposing sides of the ceramic sheet 64, and the lead portions 69a and 70a are exposed at the respective opposite sides. Similarly, the capacitor patterns 71 and 72 also extend to the mutually opposing surfaces of the ceramic sheet 65, and the lead portions 71a and 72a are exposed at the respective opposite sides.
With this arrangement, for example, as shown in FIG. 11, when the sheets 61 to 68 are laminated, if the sheets 64 deviate from the sheet 65 in a direction A, an area in which the capacitor pattern 71 and the open end 69b of the inductor pattern 69 defining the capacitor C1 face each other decreases, whereas, in contrast, an area in which the capacitor pattern 72 and the open end 70b of the inductor pattern 70 defining the capacitor C2 face each other increases. As a result, since the resonant frequency of the LC resonator 1 shifts in a direction opposite to a direction in which the resonant frequency of the LC resonator 2 shifts, the characteristics of the band pass filter are deteriorated.
In order to overcome the problems described above, preferred embodiments of the present invention provide a band pass filter in which the resonant frequencies of the LC resonators shift in the same direction when laminated layers deviate from each other.
One preferred embodiment of the present invention provides a band pass filter including a laminated body having a laminated body of a plurality of insulation layers, a plurality of inductor patterns, and a plurality of capacitor patterns; a plurality of inductors provided inside of the laminated body by the plurality of inductor patterns; and a plurality of capacitors provided inside of the laminated body by disposing the capacitor patterns to be opposed to the inductor patterns such that a plurality of LC resonators are formed thereby. In this band pass filter, the capacitor pattern of a first LC resonator of at least one pair of the adjacent LC resonators and the inductor pattern of a second LC resonator of the pair of the LC resonators are disposed on a surface of a first insulation layer, whereas the inductor pattern of the first LC resonator of the pair of the LC resonators and the capacitor pattern of the second LC resonator of the pair of the LC resonators are disposed on a surface of a second insulation layer. On each of the first and second insulation layers, the capacitor pattern and the inductor pattern extend to the same side of the insulation layer so as to be exposed.
With the above arrangement, the inductor patterns of at least one pair of the adjacent LC resonators are arranged in a direction opposite to each other, and the capacitor patterns thereof are also arranged in a direction opposite to each other. As a result, the adjacent LC resonators are interdigitally coupled. In addition, since the capacitor pattern and the inductor pattern on each insulation layer extend to the same side of the insulation layer so as to be exposed, even though the laminated sheets deviate and are located at different positions, the amounts of changes in areas in which the capacitor patterns and the inductor patterns defining each of the capacitors of the adjacent LC resonators face each other are substantially equal. This allows the resonant frequencies of the LC resonators to shift in the same direction, with the result that the characteristics of the band pass filter are stabilized.
Furthermore, at least three insulation layers having the inductor patterns and the capacitor patterns disposed thereon are preferably laminated so as to increase the capacitance of the capacitor in each LC resonator. In addition, at least three LC resonators may define a band pass filter of three or more stages.
Other features, characteristics, arrangements and advantages of the present invention will become more apparent from the detailed description of preferred embodiments of the present invention with reference to the attached drawings.